I recently built a large sump for my mixed reef tank in my science classroom. I have a CPR CS100 overflow, 1" spa flex lines, and a reef run 5500 return pump. The overflow/sump input has a ball valve (soon to be replaced by a gate valve). The return line has a check valve just above the pump. See the schematic and images below. I cannot get a consistent flow rate through my system. The water level in the return chamber is constantly changing. Originally I was getting a lot of air through the overflow. I closed the ball valve a little bit and was able to get a consistent water level in the overflow chamber and stopped the air slurping. I have been adjusting the return pump power to try to match the flow rate of the overflow and the return. I can get the system relatively stable and it will stay there most of the day, but still varies around 1/2". Then, overnight the return chamber level drops significantly for no apparent reason. I suspect the outflow through the overflow is inconsistent. But, I am not sure how to get it to stabilize and stay there. This makes my ATO system useless. I am having a lot of trouble figuring this out. If you have any suggestions, I would really appreciate it! My next step is to shorten the overflow line to make a more direct travel path from the overflow to the sump. But, there is an electrical outlet in the pathway. I don't want the overflow hose to be above the outlet in case of leaks.